An asteroid that measures 400-metres wide is set to pass by Earth on Tuesday, closer to it than the moon.
The aircraft carrier-sized asteroid, named 2005 YU55, will not pose any danger to Earth, but will be visible via telescope.
It will travel past Earth at a distance of 201,700 miles (325,000km). Its closest approach will be at 23:28 GMT (3:28 pm PST) where it will be our closest encounter with an asteroid for 30 years.
"It is not going to hit us," said Marina Brozovic of NASA.
"We have a good idea of its orbit for the next 100 years and there is no chance of impact."
The next close call with an asteroid of this size will not occur until 2028.
"This is the closest approach by an asteroid that large that we've ever known about in advance," said Lance Benner of Nasa's Jet Propulsion Laboratory.
"2005 YU55 cannot hit Earth, at least over the interval that we can compute the motion reliably - which extends for several hundred years," he said.
NASA have also said that 2005 YU55 will have no gravitational influence on Earth either so will not cause any earthquakes or tsunamis.
NASA have suggested that amateur astronomers may be able to catch a glimpse of the asteroid with a telescope of 15cm or larger, but have warned that it will be moving so fast it could be difficult to track.
Discovered by Robert McMillan of the Spacewatch Program near Tucson, Ariz., on Dec. 28, 2005, the asteroid will be examined by researchers who will hope to analyse radar images to glean details about the asteroid's surface features and shape.